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Q. We plate carbon steel with brass, and our process consists of hydrochloric acid pickling, rinse, cyanide brass plating, or copper and zinc followed by diffusion. Would it be possible to plate stainless steel with the same process? D.V.
A. The process you describe is not the best process for plating stainless steel. Stainless steels form oxides on the surface which help protect them from corroding but also prevent good adhesion. When you plate stainless steel, removal of these oxides prior to plating is the most critical step.
There are a number of procedures that are commercially used to prepare stainless steel surfaces for plating. One method is to treat the parts in 20–50 percent sulfuric acid at 65–80°C for at least one minute after gassing starts. Another method is to treat cathodically in 5–50 percent hydrochloric acid at room temperature and a current density of 0.20 ASF. Beware that this process generates large amounts of hydrogen, however. Nickel anodes are recommended.
Another alternative is to cathodically treat the parts in a solution containing approximately 120 g/L of a powdered acid and some fluorides. A temperature of 40–50°C and carbon anodes are recommended, and a current density of 0.20 ASF is usually used.
The best method by far and the most foolproof is a Wood’s nickel strike. One formulation (there are many given in the literature) consists of 45 g/L of nickel in 10 percent by volume of commercial hydrochloric acid. The nickel is added as nickel chloride, and the bath is normally used at 10–40 ASF at room temperature for 30 sec to 2 min. Nickel anodes are used, and nickel content will increase over time. The nickel concentration should be monitored with 90 g/L as the upper limit. The process steps are: anodically clean, rinse, activate with the Wood’s nickel strike, rinse rapidly and plate.